I have noticed this growth on my dogs tail for a couple months now. I never thought anything of it, I just brushed it off thinking it was a small mat or something and had never really gotten around to brushing it because he hates that area of his tail being touched. Today I examined it more clearly and I realized that it was not a mat but some sort of growth. I am worried sick now and I am afraid it could be a tumor or cyst of some sort. My dog is my best friend and I have had him for over half of my life. He is almost eight years old and has regular Vet check ups and is an incredibly healthy dog. I just don't want anything to happen to him and I want to know if I should get this checked out by a Vet. And to see what you all thought it was. Additional information: it is sensitive to the touch and I have seen him frequently scratching and chewing that area of the tail.

Marko, it's the bump at the bottom of his tail on the picture.
And barkingdog, I saw that. But it is nothing like that. It's round and is about the size of nickel maybe even bigger. I will keep you all updated thank you for your opinions and help it is greatly appreciated.